1. Field of the Invention
The present invention relates to extruded thermoplastic polymer foam having high strength and thickness and a process for preparing such polymer foam.
2. Description of Related Art
High strength structural polymer foams have value in demanding applications such as boat hulls and windmill blades. Polymer foams are desirable for such applications because of their combination of relatively light weight yet high strength, as well as their ability to be cut, shaped and molded to desired shapes and sizes.
Despite their value, there are limited commercially available options for high strength structural polymer foams and those that are available tend to be expensive and create disposal challenges. Crosslinked polyvinylchloride (PVC) foam has been a long-standing option in this market and is available under trade-names such as Airex™ structural foam (Airex is a trademark of 3A Composites GmbH). Crosslinked PVC foam provides desirable strength and weight, but has disposal challenges. Extensive crosslinking precludes the foam from being recycled by melting and reintroducing into a foaming process. Crosslinked PVC structural foam might be able to be reprocessed to some degree by grinding it up and using it in limited amounts as filler material in some processes, but recycling it by reintroducing it into a process as a melt is not possible.
Styrene-acrylonitrile (SAN) copolymer foam is also available on the market as high strength structural polymer foam. Examples of SAN structural foam include Corecell™ brand structural foams (Corecell is a trademark of Gurit Limited Corporation). SAN structural foam offers an alternative to crosslinked PVC foam for high strength applications such as windmill blades and offers an advantage of enhanced tensile properties due to less crosslinking than the PVC alternative. However, preparing SAN structural foam is a time consuming multistep process, which is relatively expensive to operate. SAN structural foam processes are generally batch processes that require forming an initial sample of SAN polymer mixture that contains a blowing agent, pouring the mixture into metal molds and allowing it to partially cure under high heat and pressure to produce a rubbery mass (an “amoeba” or “embryo”) that is demolded and then placed into a second mold and heated again in an expansion chamber that controls the dimension in which the rubbery mass can expand so as to form a final foam having anisotropic cell dimensions. (see, for example: Sara Black, Getting to the Core of Composite Laminates, COMPOSITES TECHNOLOGY, October 2003 (available over the Internet at www.compositesworld.com/articles/getting-to-the-core-of-composite-laminates) and PCT publication WO 2009/127803 A2). The anisotropic cell dimensions and relatively high density (low void volume) of the resulting foam result in stiffness and strength properties that are desirable for structural polymer foam.
It would be desirable to identify a continuous extrusion process for preparing structural polymer foam in order to make the production process more efficient and less expensive. It would further be desirable for such a process to produce thermoplastic polymer foam that can be recycled. Use of structural foam in wind mill or wind turbine applications to produce green energy compromises the green aspect of the device if the material in the blades cannot be recycled and must be disposed of in land fills.
In particular, it would especially be desirable to identify a continuous extrusion process for directly extruding a structural thermoplastic polymer foam that has a thickness of at least 50 millimeters, a density of at least 45 kilograms per cubic meter and void volume of 96 volume-percent or less, an average shear modulus between the thickness/length (xz) and thickness/width (xy) dimensions that is greater than 16 mega pascals (MPa), a tensile modulus in the thickness (x) dimension that is greater than 35 MPa and a compressive modulus in the thickness (x) dimension that is greater than 35 MPa as a single board. Of particular interest is such a process that produces polymer foam that can be recycled by melting and introducing back into an extrusion foam process.